The invention relates to a method for register correction in machines that process webs of material.
Such a machine has transporting and processing stations, for instance with appropriate driven cylinders. In this respect, for the sake of simplicity, merely their shafts will be referred to.
Such methods are employed for instance in rotary printing presses, paper processing machines or sheet-fed printing presses, when an already-processed or -printed web of paper is to be further processed or printed (insetting), so that the subsequent processing steps must be done at a longitudinal position that is oriented precisely relative to an imprint that has already been made on the paper web. This assures that for instance two successively applied printed motifs will coincide in the predetermined relative position on the paper. To achieve this, cooperating transport shafts and processing shafts are corrected relative to one another by means of the register correction.
In machines that process webs of material, the principle has meanwhile become established that the shafts of a processing machine or part of a machine be equipped with individual drive mechanisms synchronized with one another, thus for instance replacing a mechanical vertical shaft (see for instance the documentation of SYNAX 6, 2000, put out by Rexroth Indramat GmbH). To that end, the applicable shafts (as a result of the synchronization of the associated drive mechanisms, or via higher-order controls) obey a higher-order chronological guide shaft function and are thereby synchronized. In such a context, “obey” means that the motion of the applicable shaft is derived directly from the guide shaft function, or from the guide shaft function via an (electronic) conversion. The guide shaft function corresponds to an instantaneous position of a guide shaft that is for instance virtual, that is, electronically generated, or a real guide shaft. For instance, it can reflect the course over time of the instantaneous position, that is, the angular position of the guide shaft; however, it can also include the course over time of the speed of rotation or other parameters corresponding to the instantaneous position of the guide shaft. In particular, it is an electronic, chronological sequence of set-point values.
In addition, a plurality of register-tracking shafts are corrected relative to the guide shaft function in accordance with a scan of register marks of the webs of material. These shafts are corrected in terms of their instantaneous position, their instantaneous speed of rotation, or corresponding parameters. The extent of the correction is determined by the scanning of register marks. The register marks can for instance be printed on—as is usual in the prior art—and can be scanned optically.
It is known for each shaft to be corrected to be regulated with its own register regulator. The necessity therefore arises of parametrizing each shaft and its regulator individually and to optimize them in terms of the corrective motions and the synchronicity with the other shafts. The effort upon startup is accordingly great; furnishing such a high number of individual register regulators is additionally associated with high effort for apparatus and leads to high costs. Nevertheless, the synchronicity of the shafts to be corrected is not always satisfactory, since intrinsically, mechanically and electronically caused deviations can occur between the individual register regulators. The result can be fluctuations in the tension of the web.
It is also known to have one register regulator act simultaneously on a plurality of shafts. To that end, an individual correction signal is transmitted to each shaft—that is, to the applicable drive mechanism or applicable controller of the applicable element, such as the cylinder—where it is converted into the corresponding individual corrective motion. The effort and expense for this rises sharply with the number of shafts to be regulated, so that for a large number of shafts to be regulated—which is widely the case—this method can be employed only with limitations, if at all. Problems of synchronicity can also occur because of excessively long cycle times in transmitting the correction signal.